1. Field of the Invention
This invention relates to acoustic resonators and methods for forming such devices. In particular, but not exclusively, this invention relates to an electrode of Molybdenum (Mo) upon which is deposited a piezoelectric layer for example of Aluminium Nitride (AlN). Such structures are typical of acoustic resonators formed on silicon wafers.
2. Description of the Related Art
Acoustic resonators are used as RF filters and resonators such as Bulk Acoustic Wave (BAW) or Film Bulk Acoustic Resonators (FBAR), and the term “acoustic resonator” is used broadly to cover all such devices and others using similar structures. In such devices, a piezoelectric layer of e.g. crystalline Aluminium Nitride lies between at least two electrodes. Upon application of a high frequency voltage to the structure the piezoelectric layer will vibrate in an allowed vibrational mode at a chosen frequency thereby enabling a band pass filter or frequency stabilization.
Molybdenum is used in e.g. FBAR devices as the bottom electrode as it has the advantage of lower acoustic losses, due to its higher stiffness, compared to other electrode metals like Al, Pt, Au and Ti. A benefit related to the lower acoustic losses in Mo is the higher Q factor of devices, with Q determining the speed with which the filter switches on and off.
One of the primary requirements for the good performance of FBAR devices is a sharp {0002} texture in the AlN films, with a full width, half maximum (FWHM) of the rocking curve on a preselected crystallographic plane preferably of 2.0° or less. It is known that the texture of the AlN films is strongly dependent on both the roughness and the texture of the underlying electrode upon which it is deposited. A smooth underlayer with a sharp texture is the best possible combination.
In GB-A-2 349 392 the applicants describe the use of atomic hydrogen in a plasma either prior to, during or after the deposition of the underlying electrode. This then resulted in an improved quality of an Aluminium Nitride layer subsequently deposited upon that electrode. The process was not well understood at the time and therefore not optimised.
In U.S. Pat. No. 6,060,818 it was shown that polishing the surface of a phosphorus doped silica glass (PSG) layer to a mirror finish enabled a well-collimated film of Molybdenum to be deposited. This disclosure further states that this mirror finish to the PSG layer then formed the basis for a highly textured c-axis piezoelectric layer “in spite of the fact that it does not contain a crystalline structure that “seeds” the piezoelectric layer”. The PSG layer of this disclosure is a sacrificial layer, being subsequently wet etched to create a cavity. The chemical mechanical polishing process and subsequent cleaning are both complex and expensive.
There therefore remains a need for an improved method of depositing a lower electrode suitable as a surface upon which to deposit a piezoelectric layer, but which does not require a sacrificial layer under the electrode layer and/or polishing of the immediate underlay.